<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<style>
    .wrap {
        width: 400px;
        height: 400px;
        background: #c1c1c1;
        display: flex;
        border: 1px solid #cc0000;
        align-items: center;
        justify-content: center;
    }

    .item {
        background: #fcc;
    }

    .i1 {
        flex: 0 1 auto;
        width: 100px;
        height: 100px;
        background: #fc0;
        word-break: break-all;
        /*必要*/
    }
</style>

<body>
    <div class="wrap">
        <div class="item i1">
            1111111111111111111111111111111
        </div>
    </div>
</body>

</html>